Who or Whom?

in Formal Language we use WHOM instead of WHO / THAT:

-  As the OBJECT of the Relative Clause:

That's the girl whom I met at the party.

- AFTER A PREPOSITION (That would never be used)

That's the girl about whom Paul is talking all the time.

In Informal Language we omit this Relative:

That's the girl (whom) I met at the party. 

and the preposition is placed after the verb it modifies:

That's the girl (whom) Paul is talking about all the time.

In Non Defining Relative Clauses, 

because we can NOT omit the Relative Pronoun, 

the FORMAL Structure is common:

Peter, whom I met yesterday, is working abroad.

Ms. Wilkins, with whom you are working at present, used to work with me.

Who or Whom?: Study
Practise: 1, 2
Choose the pronoun (including WhoM): 1, 2, 3
Some songs with WHOM...:
Whom shall I fear?
For whom the bell tolls? (Metallica)
Bee Gees:

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